Such bogies are used for travelling over tracks in bad condition. The greatest inequalities at least are absorbed not by the clearance of a suspension between axle and sole-bar but by an equalising movement of the sole-bar in its vertical plane.
However, this type of chassis has the drawback of permitting numerous parasitic movements and, especially, movements according to which each sole-bar has the tendency continuously to overtake the other or be overtaken by it. In oscillatory mode, this type of movement leads to a sideways-motion trajectory of the assembly of the vehicle and, possibly, a derailment.